The present invention relates to a feeding device for an imaging apparatus which carries out a printing operation on a continuous form recording sheet.
Conventionally, as an electrophotographic imaging apparatus, there have been known an electronic copy machine, a laser beam printer, and the like. In such an electrophotographic imaging apparatus, a uniformly charged surface of a photoconductive is exposed to light by an exposing unit in order to form a latent image on the basis of image information. Then, toner is stuck to the latent image by a developing unit so as to form toner image. And, in turn, the developed toner image is transferred onto a recording sheet by a transferring unit. Subsequently, the transferred toner image is finally fixed by a fixing unit.
In the above-described fixing unit, there has been generally employed a so-called heat-roll fixing device, which includes a pair of fixing rollers. The pair of fixing rollers comprises a heat roller to be heated to a predetermined high temperature zone, and a back up roller being abutted against the heat roller with a predetermined pressure.
A recording sheet bearing an unfixed toner image is inserted between the pair of fixing rollers, so that heat and pressure are applied to the recording sheet. By applying heat and pressure, the toner images are fused and fixed on the recording sheet. Thus, the fixing operation is accomplished.
In general, when the feeding operation of a recording sheet is carried out in the printer, an error of the feeding speed of the recording sheet tends to occur due to, for example, tolerance of the feeding roller's radius and expansion/contraction occurring during the feeding operation.
Especially, when the pair of fixing rollers are constituted to serve, not only as fixing rollers, but as recording sheet feeding rollers, there was a problem such that an error of the feeding speed of the recording sheet tends to occur due to the increase/decrease of the radius of the fixing rollers caused by temperature change, or abrasion or recording sheet expansion/contraction caused by temperature change.
FIG. 13 shows an example of a pair of fixing rollers 8 capable of functioning as a recording sheet feeding roller. In the drawing, a heat roller 81 is driven by a drive source 40 to rotate at a predetermined rotational speed. A back up roller 82 is rotatably abutted against the heat roller 81 with a predetermined pressure.
The heat roller 81 has a circumferential surface coated with fluorocarbon resin or applied with silicon oil in order to prevent seizure of toner or offset phenomenon. Accordingly, the circumferential surface of the heat roller 81 has an extremely low frictional coefficient.
When the feeding operation of the recording sheet is practically carried out with use of the pair of fixing rollers 8, the backup roller 82 is driven to rotate by the heat roller 81. Thus, a recording sheet is fed by the paired fixing rollers 8.
When the recording sheet 20 is fed as shown in FIG. 13, the back up roller 82 has different temperatures between a region 82B which contacts the recording sheet 20 and regions 82A which directly contact the heat roller 81.
That is, the region 82A of the backup roller 82 has a higher temperature than the region 82B does because region 82A receives heat directly from the heat roller 81. Therefore, the diameter of the region 82A of the backup roller 82 becomes larger than that of the region 82B due to difference of the amount of expansion. In other words, even if the surface speed of the cylindrical region 82A equals that of the heat roller 81, the surface speed of the cylindrical region 82B becomes slower than that of the heat roller 81, since the diameter of the region 82B is smaller than that of the region 82A.
As a result, a feeding speed of the recording sheet 20 becomes relatively slower since it is fed in accordance with the small diameter region 82B. This phenomenon is remarkably recognized in the case where the recording sheet has a narrow width.
Furthermore, in the electrophotographic printer, it is necessary to synchronize the feeding operation of recording sheet with the exposure operation. Therefore, if the feeding speed of the recording sheet varies as described above, it is feared that the image formation would not be executed in a normal conditions.
On the other hand, if the sheet feed operation is once interrupted and restarted after a certain period of time, the following problems may be caused.
During above certain period, the temperature of the backup roller 82 may be lowered. Therefore, the difference of the diameters between the region 82A and the region 82B may become small. In such a condition, if the heat roller 81 is driven to rotate at the same speed as when the print operation has been interrupted, the surface speed of the backup roller 82 may not coincide with the desirable value because the thermal expansion condition of the backup roller 82 may have been changed with respect to the previous printing condition.